Interim Geologic Map of the West Part of - Utah Geological Survey

noted by Moore and Straub (2001) and Moore and others (2004); deposited inbraided river and floodplain environments of a coastal plain by northeast-flowingrivers longitudinal to the foreland basin that tapped sources in the Cordilleranmagmatic arc and Mogollon Highlands (Pollock, 1999; Lawton and others, 2003;Eaton, 2006); Eaton and others (1999a) and Eaton (2006) reported enigmaticfossil mammals from near the base and top of the formation in Cedar Canyon thatmay be Campanian, and Lawton and others (2003) reported middle Campanianpollen from the upper part of the formation near Webster Flat (we also recoveredSantonian to Campanian pollen near Webster Flat); Jinnah and others (2009) andLarsen and others (2010) reported an 40 Ar/ 39 Ar age of 80.6 ± 0.3 Ma (Campanian)on lower Wahweap strata on the Kaiparowits Plateau; measurements from themap show that the Wahweap(?) Formation is about 800 feet (245 m) thick belowCedar Breaks National Monument and south of Blowhard Mountain; Moore andStraub (2001) measured 760 feet (230 m) of strata in Cedar Canyon that we assignto Wahweap(?). If the lower conglomerate of the Grand Castle Formation is notequivalent to the Drip Tank Member of the Straight Cliffs Formation, strata heremapped as Wahweap(?) may simply be the upper part of the Straight CliffsFormation (John Henry Member equivalent); ongoing stratigraphic studies mayhelp assess the validity of our tentative correlation to early to middle CampanianWahweap strata on the Kaiparowits Plateau, a correlation initially proposed byEaton and others (1999a) and Moore and Straub (2001).KsuStraight Cliffs Formation, upper unit (Upper Cretaceous, Santonian[?] toTuronian) – Consists of strata widely interpreted as equivalent to the SmokyHollow and John Henry Members of the Straight Cliffs Formation on theKaiparowits Plateau, which form an overall regressive sequence following the lastmarine incursion of the Western Interior Seaway (see, for example, Eaton andothers, 2001; Moore and Straub, 2001; Tibert and others, 2003). Lower, SmokyHollow-equivalent strata are slope-forming, brown and gray mudstone, shale, andinterbedded yellowish-brown fine-grained sandstone; lower part contains a fewthin coal beds, common carbonaceous shale, and several thin oyster coquina beds;on the Kaiparowits Plateau, upper contact corresponds to the top of the Calicobed, which Moore and Straub (2001, their subunit 4 of interval A) suggested maybe present in Cedar Canyon about 285 feet (87 m) above the base of theformation; this sandstone is about 30 feet (9 m) thick and is not distinctive – itlacks pebbles and we were unable to use this bed as a marker horizon; however,we did locally map a thin pebble conglomerate about 330 feet (100 m) above thebase of the formation in the southern part of the Webster Flat quadrangle, whichmay be the Calico bed; Smoky Hollow strata are middle to upper Turonian basedon a diverse assemblage of mollusks, benthic foraminifera, and ostracods fromCedar Canyon exposures (Eaton and others, 2001; Tiebert and others, 2003);Eaton and others (2001) measured 364 feet (110 m) of strata that likely belong tothe Smoky Hollow Member, the lower 167 feet (54 m) of which are brackish andan order of magnitude thicker than equivalent brackish strata on the KaiparowitsPlateau, reflecting greater subsidence rates in the western part of the foredeep56